Udder health was studied in 300 dairy herds grouped in three categories according to the bulk milk somatic cell count. In all herds, lactating cows were housed in a free-stall barn during the winter. All herds participated in a three or four-weekly milk recording system, had annual production quota between 300,000 and 900,000 kg, and were stocked with cows of the Holstein-Friesian or Dutch Friesian breeds. The incidence of clinical mastitis was not different among herds with a low (< or = 150,000), middle (151,000 to 250,000), or high (251,000 to 400,000 cells/ml) bulk milk somatic cell count. Clinical mastitis caused by Gram-negative pathogens occurred more often in herds with a low bulk milk somatic cell count. Clinical mastitis caused by Staphylococcus aureus, Streptococcus dysgalactiae, or Streptococcus agalactiae occurred more often in herds with a high bulk milk somatic cell count; however, the incidence of clinical Streptococcus uberis mastitis was not different among the three herd categories. The differences in bulk milk somatic cell count between the categories could be explained by the management practices studied. The incidence of clinical E. coli mastitis was mostly related to housing, hygiene, and milking machine. The incidence of clinical Staphylococcus aureus mastitis was associated with factors that were related to the bulk milk somatic cell count and to factors of which it was not clear whether they were a cause or effect of the high incidence of Staphylococcus aureus mastitis. The incidence of clinical Streptococcus dysgalactiae mastitis was related to nutrition, milking technique, and milking machine. The incidence of clinical Streptococcus uberis mastitis was associated with housing, nutrition, and milking machine. Two groups of farmers and herds could be differentiated. The first group was identified as 'Clean and Accurate', and the second group as 'Quick and Dirty'. The relationship between these two groups and bulk milk somatic cell count category was high. However, the relationship between the two groups and the incidence of clinical mastitis was weak.